1. Field of the Invention
The present invention relates to micromechanical accelerometers. More particularly, this invention pertains to a micromechanical accelerometer having a movable mass that forms the central plane of a differential capacitor and to a method of manufacture thereof.
2. Description of the Prior Art
Micromechanical acceleration sensors are increasingly employed in conjunction with capacitive measuring systems for high precision measurement of accelerations in the .mu.g range. Such devices are capable of detecting the smallest displacements (i.e. very small acceleration forces). The associated capacitive measuring systems are currently capable of resolution in the femtofarad (fF) range.
Capacitive readouts are preferred in micromechanical sensors of the aforementioned type due to the circuit-board fabrication of acceleration sensors etched from wafers. The measurement of differential capacitance changes is employed, in most cases, to increase sensitivity. In that method, a central plate (central wafer) moves linearly or rotationally in relation to two outer plates (a top wafer and a base wafer) and the difference between the two resulting capacitances is read out. In order to increase the measurement range considerably, electrostatic restoring methods, known in micromechanics, which utilize the field forces between the capacitor plates, are employed for restoration of movement of the central plate (i.e., the deflected mass). In such case, the restoration occurs through regulation of field strength or through digital regulation of temporal duration (e.g. pulse width regulation), as a constant field strength is applied. In all systems of the above-mentioned type that employ a capacitive readout, the lowest possible leakage capacitance is desired.
Differential capacitors of the above-identified type are frequently formed in micromechanics by two metal electrodes applied to glass plates and to a silicon disc, secured by anodic bonding between the glass plates. The movable central electrode (i.e., the mass to be deflected in the manner of a pendulum by inertia forces) is etched into the silicon disc and is electrically connected to the silicon frame surrounding it. In such systems, however, leakage capacitances are of the same order of magnitude or even greater than the useful capacitances due to the relatively large surfaces of the silicon frame in comparison to the surrounding, current-carrying components. Further difficulties are due to the fact that, in spite of great efforts, it has not yet proved possible to develop a glass material whose coefficient of expansion is matched, over a wide temperature range, to that of the preferred monocrystalline wafer material, silicon, that can be fabricated and connected by bonding to the wafer material.
In order to reduce the difficulties arising from differing coefficients of expansion, the base wafer and the top wafer are frequently constructed from silicon wafers with thin glass coatings. It is, however, disadvantageous in any glass insulation for conductor tracks for the outer electrodes to be guided through the bond margins that must guarantee a hermetic seal of the interior from the environment. Solutions to this problem involve guiding the conductor tracks in channels that are subsequently sealed by injection of plastic material. Sealing cannot be guaranteed, however, with this type of supply line, especially over relatively long periods of time and in the presence of large temperature differences.